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Tuesday, January 22, 2008

Real-Time Events

Opher reports a car accident (unplanned events again) and concludes that people need to process events in real-time and not in batch.

Congratulations to Opher on his fast reactions, and commiserations on the slower reactions of the driver behind. Clearly there are some events you have to process in real time. But I hope he is not implying that all events must be processed in real-time. When the fuel tank indicator appears, do you refuel immediately or do you wait until you reach the next gas station? How often do you have the vehicle serviced?

I think the critical question for systems designers here is to determine which are the events that call for a real-time response, and which are the events where a batch response is more appropriate.

There are also events that call for a low-latency (extremely fast) response, but don't count as real-time. For example, in the financial markets, there may be short-lived arbitration opportunities, which means you can make a lot of money if you can react within a small number of milliseconds. This is not a real-time requirement, because nobody expects you to pick up every single opportunity - just catch a reasonable number of them. (The reactions of the frog should allow it to catch just enough insects to fill its belly - but some insects escape to breed more insects. The insects get faster, and so do the reactions of the frogs.)

Surely the event infrastructure should be capable of handling any of these patterns.